1. Field of the Invention
This invention relates to the field of magnetic bearings and more particularly to arrangements for cancelling the vibrations, such as those caused by rotor imbalance or fluid-induced hydrodynamic forces, by means of a rotating force generator.
2. Description of the Prior Art
During the past twenty years, magnetic bearings have progressed from a laboratory curiosity to specialized, sophisticated applications such as spacecraft mechanisms, to more conventional machines such as compressors and machine tool spindles.
Magnetic bearings offer the advantages of very long life and high reliability by the elimination of wear and fatigue failure modes; by the elimination of lubrication supply and circulation systems, and by providing a way to avoid the single-point failure limitations of conventional bearing designs. Additionally, the absence of rotational frictional forces of magnetic bearings makes possible lower bearing power loss, higher accuracy pointing systems, high resolution instruments, and lighter weight gyros and momentum wheels.
All practical magnetic bearing systems constructed and operated to date have employed active servo control for at least one of the possible five degrees of freedom (i.e., x, y, z, .THETA..sub.x, .THETA..sub.y).
A radial magnetic bearing typically has two perpendicular or orthogonal axes, both of which are perpendicular to the axis of rotation. It is desirable that sinusoidal forces at different frequencies be generated in these axes to suppress vibrations of a machine casing on which the magnetic bearings are mounted. The sinusoidal forces on two axes of a radial bearing resulting in a rotating force are illustrated by an example shown in FIG. 1, along with a pair of parametric equations.
In some radial magnetic bearing applications, the control systems may be extended to four degrees of freedom in order to control angular vibrations such as pitching and swaying, in addition to horizontal and vertical lateral vibrations. Typical causes of the vibrations are rotor imbalance and the hydrodynamic effect of the fluid layers surrounding the rotor. These vibrations are typically of a frequency equal to the rotational frequency of the rotor or a harmonic (i.e., an integer multiple) thereof. The vibrations are typically measured with accelerometers.
In the prior art, control of vibrations of casings where magnetic bearings are mounted is typically accomplished by manipulating each acceleration measurement separately by using high-gain and narrow band or notch filters in order to generate compensating sinusoidal forces. However, the signal manipulating procedure in the prior art requires extensive empirical trial-and-error adjustment. This trial-and-error adjustment is deficient in that the stability of the magnetic bearing control axes cannot be assured in view of the superimposition of the additional continuous feedback control signal. If these feedback control signals are improperly set or otherwise out of adjustment, the resultant instability could damage the machinery. Moreover, achieving optimal vibration control by trial-and-error is time consuming, especially when several axes of control, or degrees of freedom, with cross-coupling influences are involved.